1. Field of the Invention
This invention pertains to the art of induction heating cooking apparatus and in particular to electrostatically shielding the cooking vessel in a manner to reduce capacitive coupling between the work coil and the vessel without significantly affecting the inductive coupling between the coil and vessel.
2. Description of the Prior Art
We are unaware of teachings pertaining to our invention in the art of induction cooking apparatus.
U.S. Pat. No. 3,806,688 filed Apr. 13, 1972, and the companion U.S. patent applications referred to therein disclose a circuit arrangement useful for an induction heating cooking apparatus. A work coil driven by this circuit is formed by litz wire spirally wound upon itself to form a relatively flat work coil of generally pancake shape, which underlies the glass-ceramic or refractory material cook top. Current through the work coil generates a varying magnetic field in the coil, which field passes readily through the glass-ceramic cook top and produces current and heating in the bottom of the metallic cooking vessel placed on the cook top over the coil.
We have found that a voltage can be measured between the cooking vessel and ground. We analyze this voltage as being derived from the capacitive coupling from the work coil to the cooking vessel since the coil and vessel may be regarded as the plates of the capacitor which has the glass cook top as the dielectric. While it is doubted that the voltages and currents at the frequency at which the work coil operates (about 27 KHz) would be lethal, particularly in view of the current being limited by the effective capacitance value, the voltage is sufficient to cause an annoying tingle to anyone who touches the metallic part of the cooking vessel. Such a tingle could cause one who is lifting a vessel with hot contents to drop the pan and suffer burns from the contents.
It is the aim of our invention to provide an arrangement which reduces the voltage coupled to a cooking vessel sufficiently that the likelihood of a shock or tingle by one touching the cooking vessel is substantially reduced.